1. Field of the Invention.
The present invention relates to a monochromator. In particular, the present invention relates to a monochromator with an off axis mirror for reducing the amount of stray light which strikes the diffractive surface of the monochromator.
2. Background of the Invention.
Monochromators are well known in the prior art. Their function is to isolate a selected wavelength of light from a source of illumination. The selected wavelength is used for analytical purposes such as analyzing the properties of a sample through which the light is passed.
Monochromators generally comprise a light source, an entrance slit for receiving light to be analyzed, means for separating the light into its individual wavelengths and an exit slit for selecting a desired component. Generally, monochromators of one type also include a mirror for receiving light from the entrance slit and collimating the light, a diffractive surface for dispersing the light into its individual components, a focusing mirror for receiving those components and refocusing them for presentation at an exit slit. After passing through the exit slit, the light is passed through a sample to be analyzed and directed to a detector to analyze the light. One type of monochromator that has been known in the art for many years is a Czerny-Turner Monochromator. One such Czerny-Tuner Monochromator is described in U.S. Pat. No. 5,192,981, the disclosure of which is incorporated by reference. Additional monochromators are described in U.S. Pat. Nos. 2,750,836 and 3,011,391, both issued to Fastie, the disclosures of which are incorporated by reference.
When the monochromator is use in spectroscopy, the amount of light absorption at a particular wavelength of light allows a chemist to determine how much of a particular chemical, enzyme, element, or compound is in the sample being measured. The sample is illuminated with monochromatic light, and light is either absorbed or transmitted according to the presence of a given molecular compound with the proper energy levels proportional to the wavelength of illumination. The resulting absorbance (optical density) or transmittance of the sample is measured. This seemingly simple procedure can present many challenges for the electro-optical designer, who must consider the light source, light transmission medium, spectral separation method, and finally detection requirements.
The light source used is usually a broad-spectrum source, such as the traditional two-lamp tungsten-halogen and deuterium system, or more recently, xenon flash lamp or white light emitting diodes. In one method of spectroscopy, the broadband source illuminates the sample directly before it is separated into its spectral elements. This separation is achieved using a planar or concave reflective diffractive surface, acousto-optical tunable filter, transmission diffraction grating, or even a transmission film grating. In this spectrograph configuration, the resultant absorption as a function of wavelength is often measured using a linear CCD or photo-diode array as the detection method. In a monochromator configuration, the light is first separated from the broadband source into its spectral elements, and then the monochromatic light is focused onto the sample of interest prior to detection at a single small detector.
In a monochromator, it is desirable to have light of only one wavelength pass through the exit slit and to the detector. In the past, after light has been reflected from the diffractive surface, a portion of that light was reflected back to the diffracting grating. This portion of the light is referred to as stray light. The stray light is then reflected back to the second mirror and out the exit slit. This light, because it is of a different wavelength from the analytical wavelength, is undesirable. One solution is to attempt to filter out the stray components by using some form of band rejection filter. However because of the geometry of the monochromator, stray light is caused by light at higher wavelengths than the analytical wavelength. The application of a low pass filter is then indicated. However, only high pass filters are widely available at economic prices. It is additionally possible to eliminate this source of stray light by masking off the center region of the grating. This has the undesirable side effect of drastically reducing the total energy at the analytical wavelength.
The present invention provides a novel solution to the stray light problem, without the drawbacks of the solutions used in the past.
The present invention greatly reduces the amount of stray light striking the diffractive surface of the monochromator. The invention includes a housing adapted to contain the components of the monochromator. A light source is. provided, for example, a xenon flash lamp, which is designed to direct light to a source mirror. Other light sources could be used including tungsten-halogen lamps, tungsten-halogen and deuterium lamps, or white light emitting diodes. The source mirror reflects light from the light source through a first slit, the entrance slit. Preferably, the light source directs the light at a downward angle toward the mirror. The light source is preferably in the same vertical plane as the axis through the entrance slit to minimize the effects of aberrations.
After passing through a filter and an entrance slit, light is directed to a first monochromator mirror. Alternatively, the filter may be placed in other locations in the optical path. The first monochromator mirror collimates the light and reflects it to a diffraction grating or defractive surface having a reflective diffractive surface. The diffractive surface rotates about a vertical axis, the purpose of which will be described hereinafter. The diffractive surface separates the light into its individual wavelength components. The separated light is directed from the diffractive surface to a second monochromator mirror. The second monochromator mirror directs the light to an exit slit.
In monochromators of the past, some light from the second monochromator mirror could be directed back to the diffractive surface. This stray light is then directed back to the second monochromator mirror and can be presented to the exit slit. It is desirable to have light of only a single analytical wavelength pass through the exit slit. The present invention solves the problem of stray light being directed back to the diffractive surface by tilting the second monochromator mirror off axis. Specifically, the second monochromator mirror is tilted at an angle xcex8 from the vertical. The light which is reflected from the second monochromator mirror will not strike the diffractive surface but will pass harmlessly above or below the diffractive surface, or if it strikes the diffractive surface it will not be in the plane of diffraction. Light striking the diffractive surface out of the plane of diffraction will be reflected above or below the second monochromator mirror. The angle at which the second monochromator mirror is tilted may be any suitable angle sufficient to prevent the light from striking the diffractive surface, or to prevent the light from striking the diffractive surface in the plane of diffraction. Preferably, the angle xcex8 is about four degrees when the distance between the mirrors and slits (the focal length) is about 140 mm, although other dimensions are possible. It is important that the angle xcex8 be from the vertical, i.e. that the light be directed at an angle from the horizontal so that horizontal aberrations are minimized. If the mirror is tilted with respect to a horizontal axis sufficiently to prevent light from reflecting back to the grating, aberrations in the horizontal would limit the bandpass of the monochromator. Also, because the spectrum is spread across the horizontal plane, a much smaller angle is required with respect to the vertical than if the mirror is tilted with respect to a horizontal axis. If the mirror is tilted with respect to a horizontal axis, the footprint of the monochromator would have to be much larger.
The exit slit, of course, must be at a sufficient elevation above the diffractive surface in the housing to receive the reflected light. The light passes through the exit slit to a sample mirror. From this sample mirror, light is directed through a beam splitter, as is known. A portion of this light passes through the beam splitter, through a sample to be analyzed and to a detector. The sample may be held in a cuvette as is known and may be one of several samples held to be analyzed in series to increase the throughput of the analyzer. The other portion of the light reflects off of the beam splitter and passes to a second detector. The light striking the first detector (sample detector) is compared to the light striking the second detector (reference detector) so that the properties of the sample may be analyzed. Alternatively, the reference detector could be eliminated and only the sample detector could be used.
The design presented here overcomes the problem of prior art monochromators in a simple design by tilting the second monochromator mirror upwards. The image of the slit is moved upwards which is accommodated for by moving the exit slit to meet the image. Correction of the tilted optical axis is accomplished by tilting the sample mirror downward. By tilting the second monochromator mirror sufficiently, stray light passes over the top of the grating missing it completely or is directed out of the plane of diffraction. Additionally, energy throughput remains high, and spectral imaging quality is not sacrificed because the aberrations caused by tilting the mirror up are in the opposite plane. Tilting the mirrors after the diffractive surface minimizes the stray light problem. By tilting mirrors only after the grating, collimated xe2x80x9cin-planexe2x80x9d light is still incident on the grating allowing for good spectral imaging. Also, by tilting only the second monochromator mirror out of the plane of diffraction, there are no additional aberrational contributions to the horizontal (spectral) plane and stray light is still reduced. The monochromator of the present invention also has a small foot-print.
The invention will now be described in detail with reference to the accompanying drawings.